1. Field of the Invention
The present invention relates to a digital signal detector that detects a predetermined data string among modulated digital signals and a method for detecting a digital signal. Moreover, the present invention relates to a digital demodulator for demodulating modulated digital signals and a method for synchronous detection by the digital demodulator.
2. Description of the Related Art
Recently, broadcasting systems such as television, etc., has been being shifting from an analog system to a digital system. The ISDB-S (Integrated Services Digital Broadcasting) that is one of the satellite digital broadcasting systems in Japan is a standard for transmitting signals by combinations of a plurality of modulation techniques. It is indispensable that a demodulator of the ISDB-S synchronously detects signals even where the noise field intensity is very high and maintains its synchronized state. In order to enable synchronous detection and to be able to maintain its synchronized state under these bad conditions in the ISDB-S, information necessary to demodulate transmission signals (a signal provided for the purpose of synchronous detection and maintaining its synchronized state) is arrayed in transmission signals in accordance with a predetermined rule. And, it is necessary to detect the information in order to demodulate the transmission signals.
FIG. 1 shows the outline of transmission signals in the ISDB-S.
The largest unit of a transmission signal is called a “super frame”, and one super frame consists of eight frames. One frame is composed of a synchronizing character 1, a TMCC (Time Multiplexing Configuration Control) signal, a synchronizing character 2 (or a synchronizing character 3), a main signal, a burst signal, a main signal, etc. The synchronizing characters 1, 2 and 3 are specified bit strings necessary to establish synchronization, wherein each of the bit strings is composed of 32 symbols. The TMCC signal is a signal that shows information of a transmission system and a transmitting station, etc., in one super frame, which consists of 1024 symbols (128 symbols×8 frames). The main signal is a signal that is obtained by modulating data of pictures and sound, etc., which are transmitted by a broadcasting station. The burst signal is a signal to be inserted to enable synchronization where the noise field intensity is low, and to maintain the state after the synchronization is established.
The synchronizing characters 1, 2 and 3, the TMMC signal, and the burst signal are modulated by the BPSK (Binary Phase Shift Keying). To the contrary, the main signal may be transmitted by a combination of optional modulation techniques. That is, the main signal is modulated by the BPSK, QPSK (Quadrature PSK), or 8PSK. The TMCC signal has information showing which types of modulation techniques are combined to transmit signals. Therefore, the main signal may be demodulated for the first time by acquiring the TMCC signal. That is, detection of the synchronizing characters and demodulation of the TMCC signal are required to commence the demodulation.
Thus, detection of the synchronizing characters is very important to demodulate transmitted signals. A detector that detects the synchronizing characters is required to detect synchronizing characters in poor conditions as described above.
FIG. 2 shows various phase vectors (signal point maps) of synchronizing characters modulated by the BPSK. In the drawing, the axis I indicates a carrier phase that has the same phase as that of the reference phase, and axis Q indicates a carrier phase orthogonal to the axis I. For example, information of signals modulated by BPSK is as shown in FIG. 2(a). That is, a binary state (0 or 1) is discriminated by using the axis Q as a boundary axis. Therefore, even in a case where the frequency of a carrier wave more or less shifts from the frequency of an oscillator, (for example, a carrier regenerating circuit) in a demodulator (a frequency difference), it is possible to detect synchronizing characters by the discriminator having a discriminating condition of FIG. 2(a). Herein, in the BPSK, a range fd[Hz] of the above-mentioned frequency shift where the synchronizing characters can be detected is fd<F/2N, when the length of the synchronizing characters is N[bit], and the modulating speed is F[baud].
However, where synchronizing characters are detected by only a discriminator having the discriminating conditions described in FIG. 2(a), the range fd of the frequency shift becomes smaller when a noise occurs in a transmission line. In a case where a synchronous character is on the axis Q (on the boundary between two digits) without a noise in a transmission line and with the range fd of the frequency shift being small, synchronous character can not be detected. Therefore, the conventional synchronizing character detector is provided with a discriminator having the discriminating conditions described in FIG. 2(a) and at the same time discriminators having discriminating conditions (FIGS. 2(b), (c) and (d)) which are set by shifting phases, which are to be the boundaries of discrimination, from each other. By concurrently operating four discriminators with different discriminating conditions from each other, it is possible to detect synchronizing characters regardless of phases of carrier waves and the occurrence of noise.
FIG. 3 shows a synchronizing character detector having four discriminators 2.
The discriminators 2 respectively have the characteristics shown in FIGS. 2(a), (b), (c) and (d), wherein an input signal (synchronizing character modulated by the BPSK) is binarily detected. Also, in fact, the four discriminators 2 can operate equivalent to the case of eight discriminators 2 upon receiving an input signal or an inverted input signal. Results of the discrimination made by the discriminators 2 are stored in a buffer 4 that stores information equivalent to the synchronizing character length (in this example, eight symbols). The synchronizing character discriminator 6 compares a comparison data string (input signal) stored in the buffer 4 with an expected data string (synchronizing character), and it detects a synchronizing character included in the input signal. The result of the detection of synchronizing characters, which have been made by the synchronizing character discriminator 6, is outputted as an signal of the synchronizing character via an OR circuit 8. That is, since any one of the synchronizing character discriminators 6 detects a synchronizing character, the synchronizing character detection signal is activated. And, the TMCC signal is demodulated after the synchronizing character is detected, and the transmission signal is demodulated.
FIG. 4 shows an example of a prior art synchronizing character detection.
In the example, the oscillation frequencies of a demodulator are sequentially shifted by a frequency sweeper, and the frequency shift of the carrier waves that can be demodulated by the demodulator is set larger than the range fd of the frequency differences. Figures in brackets in the drawing indicate the order of sweeping. The frequency can be swept nine times, and the synchronizing characters can be demodulated in a range of ±9 fd.
FIG. 5 shows a flow chart of the detection of a synchronizing character in a demodulator having a frequency sweeper.
First, the demodulator converts the frequency of a reference signal in a carrier regenerating circuit to a frequency corresponding to, for example, FIG. 4(1), and repeatedly detects synchronizing characters while a timer is operating. The cycle of the timer is set so that synchronizing characters are detected several times to several tens of times. By detecting the synchronizing characters a plurality of times, the synchronizing characters can be securely detected even in a case where noise occurs. Where the synchronizing characters are detected, the operation flow shown in FIG. 5 is terminated, wherein the TMCC signal and main signals are demodulated.
Where no synchronizing character is detected during the operation of the timer, the operation frequency of the carrier regenerating circuit is shifted to the next area (for example, FIG. 4(2)) by the frequency sweeper. And a synchronizing character is repeatedly detected. Where no synchronizing character can be detected, the frequency sweeping and detection of synchronizing characters are repeated in the ranges shown in FIG. 4. As a result, even in a case where a frequency shift of the carrier wave from the carrier regenerating circuit is large, it is possible to detect synchronizing characters.
However, where the noise field intensity is large, the time required to detect a synchronizing character is increased due to a decrease in the detection probability of a synchronizing character. In addition, when the noise field intensity is large, the range fd of a detectable frequency shift is decreased since a signal does not exist at a position in a phase space where the signal is to originally exist. Therefore, in order to securely detect a synchronizing character when the noise field intensity is large, it is necessary to reduce the range fd of the frequency shift and lengthen the time required for detection of a synchronizing character (the number of times detection is performed) in a range fd. In other words, even though an optimal time necessary for detecting a synchronizing character and the sweeping frequency are determined by measuring the noise field intensity, the time for detecting a synchronizing character of a signal having a specific frequency difference will be lengthened as the noise field intensity increases. Therefore, the length of time needed for detecting a synchronizing character has importance when the noise field intensity is high although it does not when it is low.
In order to solve such a problem, a plurality of carrier regenerating circuits are formed in the demodulator, and these carrier regenerating circuits are operated in parallel, synchronizing characters with a plurality of frequency differences may be simultaneously detected. However, the carrier regenerating circuit is composed of a phase rotator, a synchronizing character detector, a loop filter, a timing generator, a ROM, etc., wherein the circuit scale is large among the components of a demodulator. Accordingly, it is difficult to achieve such a carrier regenerating circuit in terms of cost and power consumption.
Further, a demodulator, in which a rotation angle of a phase of a signal that is received is calculated based on a difference between the phase of the past signal point and the phase of the present signal point, and which operates on the basis of the rotation angle, is disclosed in Japanese Patent Gazette No. 2538888. However, in case that the noise field intensity is large in the demodulator, the calculated phase rotation angle (predicted value) does not coincide with the actual rotation angle. Therefore, the signal detection performance is drastically lowered when the noise field intensity is very high. Accordingly, it is difficult for such a type of a demodulator to be employed as a demodulator of the ISDB-S.